(1) Field of the Invention
The present invention generally relates to an apparatus for controlling a heater for an oxygen sensor, and more particularly to a heater control apparatus for varying electrical power supplied to a heater provided for an oxide semiconductor type oxygen sensor fastened to an internal combustion engine so that the resistance value of the heater is always equal to a target resistance value.
(2) Description of the Related Art
Recently, various control devices have been developed which are intended to provide an improvement in output power of an internal combustion engine, a reduction of fuel consumption or reduce the amount of undesirable exhaust gas. Normally, such control devices employ oxygen sensors. As is well known, an oxygen sensor is fastened to an exhaust passage of the internal combustion engine and is used to measure the concentration of an oxygen component contained in an exhaust gas. An electronic control type fuel injection device calculates a basic fuel injection time based on the amount of intake air (or negative pressure measured in an air intake pipe) and the revolution speed of the engine, and corrects the basic fuel injection time on the basis of a sensor output signal (voltage) of the oxygen sensor so that a mixture supplied to a combustion chamber is equal to a target air-fuel ratio (for example, a stoichiometric air-fuel ratio).
An oxygen sensor has a sensor element (sense portion) formed of, for example, an oxide semiconductor or a concentration cell. An oxide semiconductor type oxygen sensor includes an oxide semiconductor, such as TiO.sub.2 (titania), which has a variable resistance based on the oxygen concentration. A concentration cell type oxygen sensor includes, for example, a zirconia element, which generates a voltage developed across opposite sides thereof when there is an oxygen concentration difference between the opposite sides.
FIG. 1 shows a sensor resistance vs. sensor temperature characteristic of an oxide semiconductor type oxygen sensor, such as a titania oxygen sensor. As shown by curve I, an oxide semiconductor sensor has a low resistance Rr when the oxygen concentration is low, that is, the mixture of air and fuel is rich. On the other hand, as shown by curve II, the oxide semiconductor sensor has a high resistance Rl when the oxygen concentration is high, that is, the mixture of air and fuel is lean. Normally, a change in the resistance of the oxide semiconductor element, labeled, R.sub.T, is not directly detected, but detected by using a circuit shown in FIG. 2. In FIG. 2, a resistor R.sub.O is connected to the oxide semiconductor element, and a battery V.sub.B is connected to a series circuit consisting of the resistance component R.sub.T of the oxide semiconductor element and the resistor R.sub.O. A change in the resistance R.sub.T of the oxide semiconductor element is detected by measuring a divided voltage (sensor output voltage) V.sub.OX developed across the resistor R.sub.O. The sensor output voltage V.sub.OX obtained when the mixture is rich is greater than that obtained when the mixture is lean.
The sensor output voltage V.sub.OX is expressed as follows: EQU V.sub.OX =V.sub.B .multidot.R.sub.O /(R.sub.O +R.sub.T) (1).
Thus, R.sub.T &lt;&lt;R.sub.O when the mixture is rich, and thus the following equation is obtained: EQU V.sub.OX =V.sub.B (high (H) level).
On the other hand, when the mixture is lean, R.sub.T &gt;&gt;R.sub.O, and thus the following equation is obtained: EQU V.sub.OX =0 (V) (low (L) level).
It is necessary for one side of the zirconia element of the concentration cell type oxygen sensor to always detect a fixed oxygen concentration because the sensor detects the oxygen concentration difference between both sides of the zirconia element. For this purpose, an air intake portion is provided in the concentration cell type oxygen sensor.
Meanwhile, as can be seen from FIG. 1, the resistance R.sub.T of the oxide semiconductor element, such as titania, depends on not only the oxygen concentration but also the temperature of the oxide semiconductor element itself (sensor temperature). Thus, it is necessary to accurately regulate the temperature of the oxygen sensor so that it is equal to an optimal temperature. For the above-mentioned reason, a heater for heating the oxide semiconductor element is provided in the oxygen sensor. Further, based on the fact that the resistance of the heater is associated with the sensor temperature, the energy supplied to the heater is controlled so that the heater resistance is equal to a target resistance value. In this way, the temperature of the oxygen sensor is controlled to an optimal temperature.
In actuality, the heaters of different oxygen sensors have different heater resistance values. Thus, even if the heater resistance value of each heater is equal to the target resistance value, the heater temperatures obtained at this time will be different from each other.
In order to compensate for differences in the heater resistance, the following control apparatus has been proposed in Japanese Laid-Open patent application No. 60-164240. The proposed control apparatus recognizes that the internal combustion engine is cold when the temperature of intake air is equal to the temperature of a coolant for cooling the engine. In this case, the heater has a thermal equivalence with the coolant. Thus, the temperature of the coolant obtained at this time is assumed to be equal to the heater temperature, and the heater resistance obtained at an absolute temperature of zero is calculated.
However, the proposed control apparatus has the following disadvantages arising from a procedure in which the learning of the heater resistance is executed while the engine is operating in a cold start state in order to calculate the heater resistance under conditions as uniform as possible.
If a defective oxygen sensor is replaced by a new oxygen sensor in a warmed-up state of the engine and the engine is started, the learning of the heater resistance may not be executed since the engine temperature has not yet decreased to a temperature obtained in the cold start state. That is, the target resistance value of the old oxygen sensor is still valid although it has been replaced by the new oxygen sensor. Thus, the sensor temperature deviates from an optimal temperature. When an air-fuel ratio feedback control procedure is executed using the new oxygen sensor in the state where the target resistance value of the old one is still valid, the air-fuel ratio calculated by the control procedure deviates from the stoichiometric air-fuel ratio, and nitrogen oxides (NO.sub.x), hydrocarbon (HC) or hydrocarbon (HC) will increase.
More specifically, when the heater resistance value of the old oxide semiconductor oxygen sensor is higher than that of the new one, as shown by 1 in FIG. 3(C) the target heater resistance value of the old sensor which is obtained by the learning and is still valid is higher than an optimal value of the new sensor. In this case, as shown by 1 in FIG. 3(B), the sensor temperature is higher than an optimal temperature, and the sensor resistance (the resistance of the oxide semiconductor element) decreases, as can be seen from FIG. 1 and the formula (1). As a result, the air-fuel ratio is controlled so that the mixture becomes lean (L), as shown by 1 in FIG. 3(A). This increases the amount of NO.sub.x, and the feedback control procedure fails in the worst case.
Meanwhile, when the heater resistance value of the old oxide semiconductor oxygen sensor is lower than that of the new one, as shown by 2 in FIG. 3(C) the target heater resistance value of the old sensor which is obtained by the learning and which is still valid is lower than the optimal value. In this case, as shown by 2 in FIG. 3(B), the sensor temperature is lower than the optimal temperature, and the sensor resistance increases, as can be seen from FIG. 1 and the formula (1). As a result, the air-fuel ratio is controlled so that the mixture becomes rich (R), as shown by 2 in FIG. 3(A). This increases the amount of HC or CO, and a catalytic smell occurs. It is now noted that FIG. 3 is a graph obtained at an ordinary temperature of 20.degree. C.
As has been described above, the conventional heater control apparatus is capable of compensating for the heater resistance differences in only a very rare operating condition "cold start state". Thus, it is impossible to compensate for heater resistance differences after the engine is started in a state other than the cold start state.